1. Field of the Invention
This invention relates to a method of preparing linear polycondensates of high molecular weight, especially linear polyesters by the further condensation of precondensates in the melted in an one-step reactor equipped with means for removing cleavage products by the application of a vacuum or partial vacuum, the reaction mass being exposed in a thin layer to the reaction conditions by means of a stirrer.
This invention further relates to an apparatus in which precondensates are further condensed to form linear polycondensates.
This invention still further relates to a novel upright multi-chamber reactor in which precondensates are formed. The invention further relates to the combination of a precondensate reactor and an one-step polycondensate reactor.
2. Discussion of the Prior Art
Both discontinuous and continuous methods are known. Discontinuous methods have the disadvantage especially that --at least in the case of production in larger units--it is not possible to overcome entirely the problems which are encountered in achieving, to melt viscosities above 4500 poises, the stirring action that is necessary to assure an adequate outgassing of the volatile cleavage products.
In conventional stirring tanks, high shear forces are produced at high rotatory speeds, which damage the melt and result in a very high specific power consumption in the stirrer motor. At low rotatory speeds, the stirring action and surface-exposing action are so minimal that the melt is exposed to great thermal stress by prolongation of the reaction.
For example, it is reported that, for the polycondensation of a polyester precondensate for a laboratory batch of 55 grams, approximately 30 minutes are required. But if one kilogram of the same precondensate is put in, about 60 to 90 minutes are required in order to attain the same degree of condensation. At the same time, maximum carboxyl group contents of 28 mass equivalents per kilogram are found in the laboratory batch, while the carboxyl group contents amount to from 60 to 80 mass equivalents per kilogram when the process is performed in large autoclaves and the polycondensation is carried to viscosities of .eta..sub.red 1.7. (Chimia 28 (1974) No. 9, pp. 544 sqq.) These high carboxyl group contents are due to the fact that, as the viscosity increases, or the degree of polycondensation increases, the speed of the reaction is more and more determined by the rate of the diffusion of the cleavage product from the surface of the polymer, and the stirring action is what determines the rate of such diffusion.
When the polyesters made by these known methods are fabricated to fibers, or injection-molded or extruded, the thermal stress that occurred during the condensation is manifested in a greater lowering of the average molecular weight, which has a negative influence on the physical properties of the plastic.
Even in the case of discontinuous production in modern horizontal reactors, the product quality is adversely affected by the relatively long time it takes to remove the product from the reactor. If the time it takes to empty the reactor amounts to 30 to 40 minutes, in the case of a polyester having a viscosity of .eta..sub.red 1.2, during this period the carboxyl group content is found to rise from 25 to 45 mass equivalents per kilogram. In the case of a polyester of a viscosity of .eta..sub.red 1.35, the carboxyl group content rises from 30 to 50 meq/kg. On the other hand, in a polyester of a viscosity of .eta..sub.red &lt;1, no increase is observed in the carboxyl group content during the emptying period. With the product removing equipment known up to the present time in large autoclaves, it is not possible to reduce the emptying time to less than stated above. Experience shows that it is possible with the methods and apparatus known hitherto to prepare polycondensates with melt viscosities up to about 4500 poises, but in the case of further polycondensation in the melt to polycondensates of higher molecular weight, product deterioration due to thermal stress is impossible to avoid.
In the preparation of linear polycondensates of higher molecular weight, especially linear polyesters from dimethyl terephthalate and alkanediols, the melt condensation is generally discontinued at viscosities of .eta..sub.red 1 to 1.2, corresponding to about 2500 to 4500 poises in the melt, depending on the temperature of the preparation process.
If higher viscosities are to be achieved, it has hitherto been customary to discharge the condensate melt from the reactor through nozzles to form strands, and to subject the cooled condensate, in the form of granules or powders of a particle size of 0.001 to 10 mm, to a solid-phase condensation. Such a process is described, for example, in German Offenlegungsschrift No. 2,315,272.
In the known continuous method of preparing polyethyleneterephthalate of high molecular weight for the production of fibers, films and the like, the process is performed in stages: the transesterification in as many as 5 stages, the preparation of the precondensate in as many as 3, and the final condensation in the melt in as many as 2, depending on the desired final viscosity. For example, in German Offenlegungsschrift No. 1,920,954, a multistage method is described for the preparation of a polyester of a medium degree of polycondensation, in which the bis-(2-hydroxyethyl) terephthalate is first prepared in a separate step. This transesterification product is made to flow, in a second step, downwardly through a series of heated reaction zones situated one above the other, while the reaction mixture is led onto each liquid surface such that the condensate is set in rotation on the tray beneath it. The precondensate thus formed (.eta..sub.red 0.5 to 0.7) is then condensed to the end in a separate reactor in a third process step.
Methods for the further condensation of precondensates, in which the reaction mass is exposed to the reaction conditions in a thin layer, are described, for example, in German Offenlegungsschrift Nos. 1,745,541 and 2,060,341, and in German Auslegeschrift No. 1,645,641. In these the reaction is performed in vertical or horizontal polycondensation reactors in which the reaction mass is exposed to the reaction conditions in a thin layer, with removal of the cleavage products, by means of a stirrer which has on its shaft, which is parallel to the tank axis, wheels spaced vertically apart and equipped with openings and scoops, individual reaction zones being provided.
In spite of the multistage process technique, polycondensates, especially polyesters of the high molecular weights that are needed in the industrial field, are not condensed all the way to the end in the melt. The melt condensation, in the preparation of polyalkyleneterephthalates of high molecular weight, is discontinued at reduced viscosities of 0.8 to 0.9 (e.g., German Offenlegungsschrift No. 2,244,664). For the achievement of polyalkyleneterephthalates of higher molecular weight, the further condensation is performed in the solid phase. A method for the solid-phase condensation of polyesters is described, for example, in German Offenlegungsschrift No. 2,315,272.
In the continuous methods for the preparation of polyesters it is stressed that the apparatus are capable mechanically of handling melts with viscosities of more than 4500 poises. Nevertheless, the limits of the known apparatus with regard to the chemistry of the processes are much lower.